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JavaScript Tutorial

Embedding JavaScript in HTML

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The HTML Document

An HTML document is made up of HTML elements, HTML element attributes, comments, special characters, and doctype. If you like to add presentational features to an HTML document you can attach css to an HTML document, to add dynamic user experience (e.g. popup, alert message, animations etc.) to an HTML document you can add JavaScipt to your HTML document.

If javascript is disabled in the browser property, even though an external script is attached or a script is written within <script>...</script> tags in an HTML document, it becomes inactive.

Certain JavaScripts do not work with all the browsers and sometimes a script works on and above (or vice a versa) a particular version of a web browser.

The script Tag

The script is an HTML element. Html script element is used to enclose client side scripts like JavaScript within an HTML document.

Syntax

<script>
  JavaScript statements....... 
  </script>

There are four types of attributes in script element:

1. language

The language attribute is used to specify the scripting language and it's version for the enclosed code. In the following example, JavaScript version is 1.2. If a specific browser does not support the said JavaScript version, the code is ignored. If you do not specify a language attribute, the default behavior depends on the browser version. The language attribute is deprecated in HTML 4.01.

Example

 <script language = "JavaScript1.2">
  JavaScript statements....... 
  </script> 

2. src

This attribute specifies the location of an external script. This attribute is useful for sharing functions among many different pages. Note that external JavaScript files contain only JavaScript statements and files must have the extension .js.

Example

 <script src = "common.js">
  JavaScript statements....... 
  </script> 

3. defer

If you set defer attribute, the browser delays the execution of the script or it changes the order of the execution of the script. This can improve the performance by delaying execution of scripts until the content of body is read and displayed by the browser.

4. type

This attribute specifies the scripting language. The scripting language is specified as a content type (e.g., "text/javascript" ). The attribute is supported by all modern browser.

Example

 <script type="text/javascript">
  JavaScript statements....... 
  </script> 

The noscript tag

If any browser does not support the JavaScript code the alternate content placed within noscript tag is being executed.

Example

 <noscript>
  ... code ....
  </noscript> 

Javascript in HTML document

There are two general areas in HTML document where JavaScript can be placed. First is between <head>......</head> section, another is specific location in <body>......</body> section. If you want to display a message 'Good Morning' (through the JavaScript alert command) at the time of page loading then you must place the script at the <head>......</head> section. In the following examples you will see the different location of <script>.....</script> tags in a HTML document.

Script in the Head

  <!DOCTYPE html>
  <head>
  <meta charset="utf-8" />
  <title> Script in head section </title>
  <script type = "text/javascript">
  JavaScript statements....... 
  </script>
  </head>
  <body>
  </body>
  </html> 

Script in the Body

  <!DOCTYPE html>
  <head>
  <meta charset="utf-8" />
  <title> Script in the Body </title>
  </head>
  <body>
  <script type = "text/javascript">
  JavaScript statements....... 
  </script>
  </body>
  </html> 

Scripts in the Head and Body

  <!DOCTYPE html>
  <head>
  <meta charset="utf-8" />
  <title> Script in head and body section </title>
  <script type = "text/javascript">
  JavaScript statements....... 
  </script>
  </head>
  <body>
  <script type = "text/javascript">
  JavaScript statements....... 
  </script>
  </body>
  </html> 

Two Scripts in the Body

  <!DOCTYPE html>
  <head>
  <meta charset="utf-8" />
  <title> Two Scripts in the Body </title>
  </head>
  <body>
  <script type = "text/javascript" scr="jsexample.js" >
  </script>
  <script type = "text/javascript">
  JavaScript statements....... 
  </script>
  </body>
  </html> 

Note : Optionally, if your script is not required to be executed before the content of the body is displayed, and your script takes longer time to load objects, you can place your script at the end of the body element.

Double or Single Quotes in JavaScript

There is no preferred method, you can use either. If you use one form of the quote (either single or double) in the string, you may use other as the literal.

* To run the code mouse over on Output panel and click on 'Run with JS' button.*

Example: Double or Single Quotes in JavaScript



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